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Reprinted from THm AMERICAN JOURNAL OF PHYSIOLOGY 
Vol. 68, No. 2, April, 1924 


AN APPARATUS FOR ELICITING AND RECORDING THE 
PATELLAR TENDON REFLEX 


W. W. TUTTLE 
From the Department of Physiology, Ohio State University 
Received for publication January 18, 1924 


It is evident that if one is to study the patellar tendon reflex quantita- 
tively it should be elicited by a stimulus of uniform intensity at regular 
intervals. Since this reflex is subject to variations due to very slight 
environmental changes it is desirable to have an apparatus which is 
capable of delivering stimuli automatically. With these ideas in view 
the apparatus described in this paper has been developed and thoroughly 
tested. The results obtained aré so satisfactory that it seems desirable 
to give a rather detailed description of it. 

The apparatus consists of four parts as follows: 1, the power unit; 
2, the hammer unit; 3, the chair unit; 4, the recording unit. 

The accompanying photograph shows the units assembled. as they 
are used in carrying out the various experiments which have been per- 
formed. | 

The power unit. The first point to be considered in constructing the 

power unit is the frequency with which the stimuli are to be delivered. 
Before deciding upon an interval between stimuli best suited to the 
elicitation of the reflex, the time was observed which was necessary for 
the reflex to be completed. It was found that from 8 to 10 seconds were 
adequate for the completion of the reflex, allowing at the same time 
sufficient rest to obviate fatigue as a significant factor. Figure 2 is a 
working drawing of the power unit. 
‘ The desired frequency is secured by means of a system of pulleys to- 
gether with resistance in the motor circuit. A 1750 R.P.M. constant 
speed motor, A, equipped with a steel pulley, B, is belted to a wood 
pulley, C; the shaft of the latter is fitted with a steel pulley, D, which is 
connected to a system of belt-driven reducing pulleys, H, fF, G. In order 
to increase the range of speeds which can be obtained a parallel bank of 
lamps, H, is placed in series with the motor.) (If desirable, a sliding 
resistance may be substituted for the lamp bank.) By varying the 
number of lamps in circuit the interval between stimuli can be made to 
vary from 4 to 12 seconds. 

In order to change the circular to longitudinal motion an eccentric, J, 
is mounted upon the last pulley. An eccentric rod, J, is attached to a 

338 


APPARATUS FOR ELICITING AND RECORDING PATELLAR RERLEX 339 


‘To Slain Lire. 


Ls 


= ad 


| 
i 


Power Ur7/C 
Tee Vrew Sea/e ay 


Reduced Y2 


Fig. 2 


340 W. W. TUTTLE 


carriage, A, which slides on a double track. It is held in place by a 
support at each end, running under the track. ‘ 

A wood bar, L, is hinged to the upper surface of the carriage well to- 
ward the end to which the eccentric rod is attached. Upon the distal 
end of this bar a hook is attached for engaging a tripping device upon the 
hammer handle. The engaging bar is firmly supported by a deep grooved 
guide at the extremity of an iron bracket fastened to the end of a table. 
The power unit complete is mounted upon a small table. 


a Hammer Onxil 
ie Saree ee front Yew 


Seale = Scale FZ 


Fig. 3 


The hammer unit. In constructing the hammer unit the principal 
points to be considered are adjustability and adequacy of striking force. 

A steel hammer, A, weighing 824 grams is suspended upon a handle 
from an axle, B. The hammer handle passes through an axle, being 
securely held in place by a thumb screw. This arrangement makes the 
length of the hammer handle adjustable. The axle swings in a bracket 
provided with two brass bearings which are adjusted so as to offer as 
little resistance as possible. 

In order that the hammer may swing automatically an engaging and 
tripping device is attached to the hammer handle. This device consists 
of a flattened metal sleeve, C, which is held in place upon the hammer 
handle by a thumb screw. To one side of the sleeve a brass quadrant, 
D, is fastened which presents a flat side parallel to the engaging hook. 


APPARATUS FOR ELICITING AND RECORDING PATELLAR REFLEX 341 


Near the central angle of the quadrant a lug, /, with a freely moving 
brass sleeve is bolted perpendicular to the quadrant. This serves as an 
attachment for the engaging hook, F. A slot is cut in the quadrant 
near its circumference and parallel to it. Opposite the engaging hook 
a piece of metal the width of the hook, but somewhat shorter, is fastened 
to the arm. This serves as a guide so that when the hook engages the 
lug the quadrant passes between the guide on the one side and the hook 
on the other. A thumb screw passing through the sleeve to which the 
quadrant is fastened holds the engaging and tripping device firmly in 
position on the hammer handle. A metal strip, G, attached to the quad- 
rant by means of a thumb screw serves as the tripping device. When 
the lug is engaged and the hammer pulled back the metal trip comes in 
contact with the inferior side of the hook arm, raising, and thus disen- 
gaging it, as the hammer continues to move toward the power unit. 
The hammer is then free to fall. | 

The power unit is placed at such a distance from the hammer unit 
that when the wood carriage reaches the distal extent of its excursion the 
hook on the hook arm drops over the lug on the quadrant. As it ap- 
proaches the proximal end of its excursion the hammer is released and is 
free to fall against the patellar tendon. 

It was found that when the hammer struck the tendon it rebounded, 
thus delivering a series of stimuli of diminishing force. This is objec- 
tionable since it not only serves as a source of annoyance but in case the 
subject is hyperirritable the first or second rebound may elicit the re- 
flex. These secondary stimuli might obviously serve to augment or 
depress the desired reaction. In order to prevent this rebound an elec- 
trically controlled brake is mounted on the axle from which the hammer 
swings. A fiber drum, H, is forced tightly on the axle near its center; 
around the drum a band spring is placed. One end of the spring is se- 
curely fastened to a brace while the other end is attached to a soft iron 
bar which serves as an armature. The spring covers about two-thirds 
of the drum. Two electro-magnets, J, mounted upon a. cross-bar behind 
the hammer draw the armature forward, thus tightening the spring and 
preventing a rebound of the hammer. The distance from the magnets 
to the armature is adjustable. In practice the adjustability of the mag- 
nets was found to be important. 

The brake is needed an instant after the hammer strikes. This is 
determined by a timing device (MV, fig. 2) attached to the axle of the 
eccentric pulley. The timing device consists of a wood pulley encircled 
by two brass strips, one of which completely encircles the pulley and 
through its connection with a copper brush serves as a constantly engaged 
terminal. The brush of the other terminal slides upon a second brass 
band partially encircling the pulley. The two bands are connected by 


342 W. W. TUTTLE 


a metallic bridge. The gap in the second band is so placed as to give 
a break of the desired length and at the desired time, while the armature 
is passing over the dry wood of the pulley. The current is kept inter- 
rupted through most of the circuit by way of avoiding heating of the 
brake magnets. One-hundred-ten volts are used in all circuits. The 
strength of the magnets is controlled by a parallel bank of lamps (JN, fig. 
2) in series with the timer. A sliding resistance might be substituted 
for the lamp bank. The entire mechanism is suspended from a cross- 
bar firmly fastened to two uprights by thumb screws. 


Platform For. Cha/r 
Front View. 
Sea/e ft 


Sea/e $ 


Feeording U0 
Fro Krew 


SIN 0 


leg Susport 
; Le Suppor i 
3 sie wew Lge ew 
aS SealeZz 
Reduced“ 


Recording Unit 
Side View" 
Seafe 


Heer Suz re ge or Stirrup 


Fig. 4 


The hammer unit includes three adjustments for varying the intensity 
of the blow: a, changing the length of the hammer handle; b, raising or 
lowering the engaging quadrant; c, adjusting the trip in the slot. The 
adjustability of the cross-bar from which the hammer swings makes it 
possible to compensate so that the height of the engaging arm remains 
constant. 

The chatr unit. The subject is seated in a barber’s chair from which 
the foot-rest was removed. ‘The necessary tension for the patellar tendon 
is secured by elevating the thigh somewhat above the horizontal and 
holding the leg slightly back of the vertical by its attachment to a spring 
that forms part of the recording device. The lower end of the thigh 
rests in a comfortable holder, at the distal end of which is a U-shaped 


APPARATUS FOR ELICITING AND RECORDING PATELLAR REFLEX 343 


guide (A, fig. 4) so constructed that both the width of the U and the 
angles of its sides can be adjusted. Although this permits more or less 
forced lateral displacement of the leg, it is found in actual practice quite 
adequate to keep the point upon which the hammer impinges in a constant 
position. 

The chair unaltered furnishes up, down and lateraladjustments. ‘To 
permit forward and backward adjustments the chair unit is mounted 
upon a double base (B and C, fig. 4), between which are four steel balls 
running in an iron groove. Wood screws in front and behind maintain 
the adjustment. 

The recording untt. <A stirrup, D, is attached by a set screw to the 
heel of the subject’s shoe. From the stirrup runs a wire to the vertical 
arm of a triangular device, H, suspended at its right angle. ‘To the hori- 
zontal arm of the triangle a second piece of piano wire is attached which 
runs to a stylus, /, suspended from a wire spring, G. The triangle pro- 


) 


Fig. 5. A record showing the constancy of the intensity of the stroke and the 
regularity with which the hammer falls. 


vides a means of changing the relation of the excursion of the heel to 
the excursion of the stylus. In practice it has been found desirable to 
work with relatively low kicks to permit registering the marked augmen- 
tations frequently encountered. 

The stylus spring plays an important role in bringing the heel back 
after each excursion exactly to its original position. By changing the 
tension on the spring the resistance offered to the reflex may be varied. 

The wire from the horizontal arm of the triangle passes through a 
screw eye at H (not shown in drawing) and under a pulley, J. The 
pulley serves as a guide so that the stylus writes perpendicularly on the 
drum. <A stop is clamped on the wire just below the screw eye to pre- 
vent fling. This device serves also to keep the initial tension constant. 

Time markers and signal magnets may be added as occasion demands. 
The entire unit is mounted on a heavy pedestal. 


344 W. W. TUTTLE 


Constancy of intervals and force of strokes. The constancy of the in- 
tensity and the regularity of the strokes are illustrated by figure 5. This 
record was made by allowing the hammer to fall against a lever hinged to 
the floor to take the place of the leg of a subject. Resistance was supplied 
by a rubber band. 

The records read from right to left due to the fact that the base line 
is at the top of the record. This method was adopted to avoid the use 
of a second pulley. 

Experience with this apparatus has shown that it is capable of eliciting 
the patellar tendon reflex to a nicety and of accurately recording any 
changes which are exhibited by it. Its complete automaticity, permit- 
ting the elimination of certain extraneous environmental variables is a 
valuable feature. Its sturdiness of construction and delicacy of per- 
formance render it a valuable addition to class room armamentarium 
both in physiology and pharmacology. 


It is a pleasure to express my appreciation for various suggestions 
received from Professors A. P. Weiss and R. G. Hoskins. I also wish 
to thank Mr. George Woodward and Mr. Carl Effler for their suggestions 
and assistance throughout the construction of this apparatus. 


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